A proteinase inhibitor based on the active site of naturally occurring proteinaceous inhibitors has been designed. This molecule is currently being synthesized and its activity as an inhibitor of various serine proteinases will be tested. Analog studies have also been planned which will allow this molecule to be tailored for greater specificity towards the active sites of various proteolytic enzymes. In particular, inhibitors of this type may be useful in regulating blood clotting or fertilization processes.